1. Field of the Invention
The present invention relates to an improvement in an MOS controlled thyristor (hereinafter referred to as an MCT) for turning on an MOS gate.
2. Description of the Related Art
FIG. 1 shows a conventional MCT, such as an MCT disclosed in JPN PAT APPLN KOKAI Publication 4-99384 (published on Mar. 31, 1992). FIG. 2 is a plan view showing diffusion layers formed in a substrate surface of an MCT of FIG. 1.
The arrangement of the MCT as shown in FIGS. 1 and 2 will be explained below.
A P type source region 1 is formed in a grid mesh-like pattern in a one-side surface of an N.sup.- type semiconductor substrate 2. P type base regions 3 are each formed in the one-side surface portion of the N.sup.- type semiconductor substrate 2 at an area between the P type source portions 3. An N type emitter region 4 is formed in the P type base region 3.
A polysilicon gate electrode 5 is formed over the N.sup.- type semiconductor substrate 2 with a gate insulating film 6 provided therebetween. One end of the polysilicon gate electrode 5 is situated in an overlying relation to the end of the emitter region 4 and the other end of the polysilicon gate electrode 5 is situated in an overlying relation to the P type source region 1.
An insulating film 7 covers the polysilicon electrode 5. A cathode electrode 8 is formed over the N.sup.- type semiconductor substrate 2 and is connected to the P type source region 1 and N type emitter region 4.
An N.sup.+ buffer layer 9 is provided on the other surface side of the N.sup.- type semiconductor substrate 2. A P.sup.+ type emitter layer 10 is formed adjacent to the N.sup.+ buffer layer 9 and an anode electrode 11 is formed on the other surface of the P.sup.+ type emitter layer 10.
FIGS. 3 and 4 show an operation (turn ON/turn OFF) principle for MCT in FIGS. 1 and 2.
Referring to FIG. 3, the operation of the MCT at a turn-on time will be explained below.
When the anode A and cathode K are biased positive and negative, respectively, with a gate G biased positive, an N channel inversion layer 12 is created at the surface of the P type base region 3. Electrons 13 are injected from the N type emitter region 4 through the N channel inversion layer 12 into the N.sup.- type base region (substrate) 2. When this occurs, holes are induced in the P.sup.+ type emitter layer 10 and injected into the N.sup.- type base region 2. As a result, a conductivity modulation occurs in the N.sup.- type base region 2 and electrons 15 pass from the N type emitter region 4 into the N.sup.- type base region 2 via the P type base region 3. In this way, the MCT is turned on.
Referring to FIG. 4, the operation of the MCT at a turn off time will be explained below.
When the gate G is biased negative with respect to the cathode K in such a state that a main current 17 flows with the anode A and cathode K biased positive and negative, respectively, the N channel inversion layer created at the turn-on time disappears. Further, a P channel inversion layer 16 is created at that surface of the N.sup.- type base region 2 situated between the P type source region 1 and the P type base region 3. Since the P type source region 1, P type base region 3 and cathode electrode 8 are short-circuited by a P channel inversion layer 16 so that holes 18 in the main current 17 are discharged from the P type base region 3 through the P channel inversion layer 16 and P type source region 1 into the cathode electrode 8. As a result, the injection of the electrons from the N type emitter region 4 into the N.sup.- type base region 2 is stopped so that the main current 17 ceases to flow. In this way, the MCT is turned off.
The above-mentioned MCT has now been developed as a self-turn-off type element with its turn-off efficiency improved in preference to other efficiencies. In the prior art, in order to improve the turn-off characteristic, the concentration of the P type base region 3 is made at as high a level as possible, thus lowering the resistance at a hole current discharge path of P type base region 3.fwdarw.P channel inversion layer 16.fwdarw.P type source region 1 as created at the turn-off time.
If the concentration of the P type base region 3 is enhanced, the N channel inversion layer as required at the turn-on time is difficult to create, thus lowering the turn-on efficiency. Stated another way, the conventional MCT suffers a drawback in that the improving of the turn-off characteristic results in the lowering of the turn-on characteristic.